1. Field of the Invention
The present invention relates to a multi-beam scanning device, a multi-beam scanning method, and a light source device for use in the multi-beam scanning device. Further, the present invention relates to an image forming apparatus in which the multi-beam scanning device is provided.
2. Description of the Related Art
With the widespread use of image forming apparatus, such as laser printers and digital copiers, there is an increasing demand for improved printing speeds of the image forming apparatus. To meet the requirement, a multi-beam scanning device having a plurality of light sources is proposed for use in the image forming apparatus. In the multi-beam scanning device, the plurality of light sources, such as laser diodes, are used to emit multiple light beams for scanning the scanned surface with the multiple light spots at a time. For example, a multi-beam scanning device using a semiconductor laser array as the plurality of light sources is known.
When the optical scanning is performed on a scanned surface of a photosensitive medium at a higher density (e.g., above 600 dpi) with the multi-beam scanning device, the pitch of the scanning lines is decreased to a smaller level to achieve the high-density scanning, and the diameter of beam spots on the photosensitive medium surface in the sub-scanning direction must be decreased accordingly. A beam spot is formed on the scanned surface by the focused laser beam from the light source. The scanned surface that is actually scanned with the focused laser beam does not necessarily accord with an image surface where the beam spot is precisely formed at the waist of the laser beam due to the field curvature of the focusing lens or the like of the optical scanning device. The diameter of the beam spot on the scanned surface is not necessarily equal to the beam waist diameter. To reduce the variation of the beam spot diameter as much as possible, the correction of the field curvature of the focusing lens is carried out.
Further, optical scanning devices after assembly contain several kinds of errors, regardless of whether they are the multi-beam scanning device or the single-beam scanning device. Such errors include respective errors of the component parts of the optical scanning device, an assembly error of the optical scanning device, and others. When such errors exist in the optical scanning device, the beam spot formed on the scanned surface with the light beam from the optical scanning device is in a defocus state. The scanned surface that is actually scanned with the focused laser beam is liable to variations of the image surface.
By taking the above matters into consideration, it is necessary that the designing of a multi-beam scanning device be based on the assumption that the beam spot on the scanned surface is in a defocus state. In most cases, the multi-beam scanning device is designed such that the variations of the beam spot diameter fall within a range of xc2x110% of a target beam spot diameter xe2x80x9cWxe2x80x9d. Namely, the beam spot diameter, which is provided by the multi-beam scanning device, is in a range from 0.9 W to 1.1 W, where W is a target beam spot diameter.
When the beam spot on the scanned surface is in a defocus state, the beam spot diameter is larger than the beam waist diameter. In designing the optical systems of the multi-beam scanning device, the practical measure is to determine a permissible beam spot diameter that is smaller than the target beam spot diameter by 1 to 10 percents. A range of the defocus amount in which the variations of the beam spot diameter are less than the above-mentioned permissible beam spot diameter is called a depth clearance. When the depth clearance is large, the degree of allowance for the variations of the scanned surface to the image surface is high. It is known from practical experience that the depth clearance that is above 0.9 mm is needed to eliminate the component part errors or the assembly errors.
Moreover, in the conventional multi-beam scanning device, the divergence angle of laser beams emitted by the semiconductor laser array is liable to variations. Generally, a semiconductor laser as in the semiconductor laser array emits a divergent laser beam. The divergence angle is at the maximum in the direction of thickness of the activated layer of the semiconductor laser and at the minimum in the direction perpendicular to the activated layer. The far-field pattern of such laser beam is in the form of an ellipse having a major axis parallel to the direction of thickness of the activated layer. In the semiconductor laser array described above, the respective divergence angles of the laser beams emitted by the plurality of light emitting parts are not common, and the divergence angle for each of the plurality of light emitting parts is liable to variations. Hence, the diameters of beam spots, which are formed on the scanned surface by the conventional multi-beam scanning device, are also liable to variations due to the variations of the divergence angles. This causes the degradation of the quality of a reproduced image.
In order to overcome the above-described problems, it is an object of the present invention to provide a multi-beam scanning device that ensures adequate depth clearance even when the optical scanning is performed at a high density above 600 dpi, and effectively reduces the variations of the beam spots on the scanned surface to a smallest possible level so that the multi-beam scanning is carried out with accurate beam spot diameter so as to create good quality of a reproduced image.
Another object of the present invention is to provide a multi-beam scanning method that ensures adequate depth clearance even when the optical scanning is performed at a high density above 600 dpi, and effectively reduces the variations of the beam spots on the scanned surface to a smallest possible level so that the multi-beam scanning is carried out with accurate beam spot diameter so as to create good quality of a reproduced image.
Another object of the present invention is to provide a light source device for use in a multi-beam scanning device that ensures adequate depth clearance even when the optical scanning is performed at a high density above 600 dpi, and effectively reduces the variations of the beam spots on the scanned surface to a smallest possible level so that the multi-beam scanning is carried out with accurate beam spot diameter so as to create good quality of a reproduced image.
Another object of the present invention is to provide an image forming apparatus in which a multi-beam scanning device is provided, the multi-beam scanning device ensuring adequate depth clearance even when the optical scanning is performed at a high density above 600 dpi, and effectively reducing the variations of the beam spots on the scanned surface to a smallest possible level so that the multi-beam scanning is carried out with accurate beam spot diameter so as to create good quality of a reproduced image.
The above-mentioned objects of the present invention are achieved by a multi-beam scanning device comprising: a semiconductor laser array which has a plurality of light emitting parts emitting multiple laser beams; a rotary deflector which deflects the laser beams emitted by the light emitting parts of the semiconductor laser array; and a focusing optical system which focuses the deflected laser beams from the rotary deflector onto a scanned surface to form a plurality of beam spots that are separated on the scanned surface in a sub-scanning direction, the scanned surface being scanned simultaneously with the plurality of beam spots in a main scanning direction by a rotation of the rotary deflector, wherein the laser array is configured such that the light emitting parts are arrayed along a line that is at an inclination angle xcfx86 to the sub-scanning direction, the inclination angle xcfx86 measured in degrees and meeting the conditions 0xe2x89xa6xcfx86 less than 90, and that a scanning line pitch P, an array pitchxcfx81 of the light emitting parts of the laser array and a parameter K defined by the equation K=0.82 xcex/xcfx89z, where xcex is a wavelength of the emitted laser beams and xcfx89z is a target beam spot diameter in the sub-scanning direction, satisfy the following conditions:
0.01 less than Kxc2x7P/(xcfx81xc2x7cos xcfx86) less than 0.30 
0.011 less than K less than 0.030. 
The above-mentioned objects of the present invention are achieved by a multi-beam scanning method that comprising the steps of: providing a semiconductor laser array having a plurality of light emitting parts emitting multiple laser beams; providing a rotary deflector deflecting the laser beams emitted by the light emitting parts of the semiconductor laser array; focusing the deflected laser beams from the rotary deflector onto a scanned surface to form a plurality of beam spots that are separated on the scanned surface in a sub-scanning direction; and scanning the scanned surface simultaneously with the plurality of beam spots in a main scanning direction by a rotation of the rotary deflector, wherein the laser array is configured such that the light emitting parts are arrayed along a line that is at an inclination angle xcfx86 to the sub-scanning direction, the inclination angle xcfx86 measured in degrees and meeting the conditions 0xe2x89xa6xcfx86 less than 90, and that a scanning line pitch P, an array pitchxcfx81 of the light emitting parts of the laser array and a parameter K defined by the equation K=0 82 xcex/xcfx89z, where xcex is a wavelength of the emitted laser beams and xcfx89z is a target beam spot diameter in the sub-scanning direction, satisfy the following conditions:
0.01 less than Kxc2x7P/(xcfx81xc2x7cos xcfx86) less than 0.30 
0.011 less than K less than 0.030. 
The above-mentioned objects of the present invention are achieved by a light source device for use in a multi-beam scanning device, the light source device comprising: a semiconductor laser array which has a plurality of light emitting parts emitting multiple laser beams; a coupling lens which couples the laser beams emitted by the laser array; and an aperture stop which restricts a diameter of the laser beams passed through the coupling lens, wherein the multi-beam scanning device comprises: the light source device; a rotary deflector which deflects the laser beams emitted by the light emitting parts of the laser array; and a focusing optical system which focuses the deflected laser beams from the rotary deflector onto a scanned surface to form a plurality of beam spots that are separated on the scanned surface in a sub-scanning direction, the scanned surface being scanned simultaneously with the plurality of beam spots in a main scanning direction by a rotation of the rotary deflector, wherein the laser array is configured such that the light emitting parts are arrayed along a line that is at an inclination angle xcfx86 to the sub-scanning direction, the inclination angle xcfx86 measured in degrees and meeting the condition 0xe2x89xa6xcfx86 less than 90, and that a scanning line pitch P, an array pitchxcfx81 of the light emitting parts of the laser array and a parameter K defined by the equation K=0.82 xcex/xcfx89z, where xcex is a wavelength of the emitted laser beams and xcfx89z is a target beam spot diameter in the sub-scanning direction, satisfy the following conditions:
0.01 less than Kxc2x7P/(xcfx81xc2x7cos xcfx86) less than 0.30 
0.011 less than K less than 0.030 
and wherein the aperture stop is configured to have a numerical aperture NAzS in the sub-scanning direction that satisfies the conditions: 0.01 less than NAzS less than 0.30.
The above-mentioned objects of the present invention are achieved by an image forming apparatus in which a multi-beam scanning device is provided, the image forming apparatus forming an electrostatic latent image on a scanned surface of a photosensitive medium through an exposure of the photosensitive medium to an imaging light pattern provided by the multi-beam scanning device, the multi-beam scanning device including: a semiconductor laser array which has a plurality of light emitting parts emitting multiple laser beams; a rotary deflector which deflects the laser beams emitted by the light emitting parts of the semiconductor laser array; and a focusing optical system which focuses the deflected laser beams from the rotary deflector onto a scanned surface to form a plurality of beam spots that are separated on the scanned surface in a sub-scanning direction, the scanned surface being scanned simultaneously with the plurality of beam spots in a main scanning direction by a rotation of the rotary deflector, wherein the laser array is configured such that the light emitting parts are arrayed along a line that is at an inclination angle xcfx86 to the sub-scanning direction, the inclination angle xcfx86 measured in degrees and meeting the conditions 0xe2x89xa6xcfx86 less than 90, and that a scanning line pitch P, an array pitchxcfx81 of the light emitting parts of the laser array and a parameter K defined by the equation K=0.82 xcex/xcfx89z, where xcex is a wavelength of the emitted laser beams and xcfx89z is a target beam spot diameter in the sub-scanning direction, satisfy the following conditions:
0.01 less than Kxc2x7P/(xcfx81xc2x7cos xcfx86) less than 0.30 
0.011 less than K less than 0.030. 
In the multi-beam scanning device and method of the present invention, the semiconductor laser array is used as the plurality of light sources and it is possible to ensure adequate depth clearance when the optical scanning is performed at a high density. The multi-beam scanning device and method of the present invention are effective in reducing the variations of the beam spots on the scanned surface, so that the multi-beam scanning is carried out with accurate beam spot diameter so as to create good quality of a reproduced image. Therefore, the image forming apparatus in which the multi-beam scanning device of the present invention is provided can create good quality of a reproduced image.